Conventionally, a semiconductor chip is die-bonded to a die pad of a lead frame, the semiconductor chip is bonded to leads radially disposed around the die pad with gold wires or the like by means of wire-bonding, and the semiconductor chip and the bonded portion are sealed by synthetic resin molding (hereinafter referred to as "molding"), whereby a semiconductor device is formed. FIG. 3 shows an example of such conventional semiconductor devices wherein leads are arranged around a die pad of a lead frame.
In FIG. 3, numeral 1 denotes a die pad; numeral 2 denotes leads; numeral 3 denotes a semiconductor chip; numeral 4 denotes a preform material comprising silver paste, Au--Si, solder, glass or the like, which is used as a material for die-bonding in order to bond the semiconductor chip 3; and numeral 5 denotes a gold for electrically connecting an electrode pad of the semiconductor chip 3 to each of the leads 2. When the semiconductor chip is fitted to the lead frame, the preform material 4 is firstly applied to the central portion of the die pad 1 of the lead frame, and the semiconductor chip 3 then bonded to the die pad by pressing it from the upper side. Since the semiconductor chip 3 is generally formed by cutting a semiconductor wafer, typically it formed in a square, and the shape of the die pad 1 typically a square greater than the semiconductor chip 3. If an amount of the preform material used for bonding the semiconductor chip 1 is too small, the strength of the bonding between the semiconductor chip 3 and the die pad 2 is insufficient. This brings separating of the semiconductor chip in the course of the production process, deteriorating of the characteristics due to an increase in electrical resistance, and deteriorating of the characteristics of the semiconductor chip 3 due to an increase in temperature which is caused by poor thermal conduction. If amount of the preform material 4 is too large, the preform material 4 is squeezed out from a rear face of the semiconductor chip 3, as shown in FIG. 3. In this case, since the preform material 4 is applied in a circular form to the central portion of the die pad 1, the preform material 4 is circularly spread on pressing the semiconductor chip 3 and is thus squeezed out from a central portion of each side of the semiconductor chip 3, without being squeezed out from the corners thereof.
As described above, a sufficient amount of preform material 4 must be applied for securing the bonding of the semiconductor chip 3. However, there has recently been the tendency that the distance between the semiconductor chip 3 and the peripheral wall of the die pad 1, and the distance between the die pad 1 and the leads 2 are decreased with a recent increase in degree of integration and a recent decrease in size of a semiconductor device. Thus, the preform material 4 squeezed out from the periphery of the semiconductor chip 3 is undesirably extended beyond the die pad 1 and comes into contacts with the leads 2, thereby an accidental short circuit occurs.
In order to prevent the preform material 4 from contacting with the leads 2 even if the preform material 4 is squeezed out from the central portion of each side of the semiconductor chip 3 and is extended beyond the die pad 1, and to improve workability of wire bonding to each lead 2, there has been employed a lead frame wherein lines connecting the tips of the leads 2 are curved so that the tips of the leads 2 are apart from portions where the preform material 4 is mostly squeezed out, as shown in FIG. 4. However, if the distance between the tip of each lead 2 and the peripheral wall of the die pad 1 is non-uniform, stress is applied by resin to the corners of the die pad 1 during resin molding. Thus, there arises a problem that a failure such as shift of the leads 2, wire distortion and the like occurs particularly at the corners, or that the die pad is shifted. With a decrease in size of semiconductor devices and an increase in degree of integration thereof, that tendency remarkably arises in a narrow-pitched semiconductor device wherein the leads are made thin, and the pitch between the leads 2 is decreased.